This invention relates to an image-processing apparatus for converting an input image with multiple gray levels to a bilevel output signal consisting of black and white dots, more particularly to such an apparatus that converts a page consisting of texts and gray-scale images into bilevel output with clean edge reproduction.
In scanning devices such as facsimile machines, a variety of schemes have been used to convert photographs and other images with multiple gray levels to bilevel output images having only two levels: black and white. A simple method is to binarize the input image against a fixed threshold, converting levels of gray that equal or exceed the threshold to black and other levels to white. Such uniform thresholding is highly unsatisfactory, because it completely fails to represent continuous shades of gray and results in great loss of detail.
A widely adopted alternative method is to dither the thresholds by varying them at individual pixel positions. This is normally done with a dither matrix such as a four-by-four matrix of threshold values, which is applied to four-by-four pixel blocks in a repeated manner to cover the entire input image. Such dithering can reproduce gray-scale images with considerable success. When the input image comprises bilevel portions such as line graphics and text, however, dithering creates unwanted effects such as blurred or serrated edges.
A prior-art solution to this problem, known as the block adaptive thresholding method, is to identify blocks of pixels as either bilevel or gray-scale in nature and apply a fixed threshold or dither matrix accordingly, as described in the paper "Bilevel Rendition Method for Documents including Gray-Scale and Bilevel Image" by N. Tetsutani and H. Ochi in The Transactions of the Institute of Electronics and Communication Engineers of Japan. Vol. J67-B No. 7, July 1984 (in Japanese).
A problem in this prior-art solution is that blocks may be mis-identified. In particular, when an all-white block is disposed next to an all-black block, both blocks may be mistakenly identified as gray-scale blocks, so some blurring of edges still occurs.